Investigations On Broadband Metamaterials Absorber And Polarization Converter

Electromagnetic metamaterial is an artificial material with many extraordinary physical properties,such as negative refractive index,negative permittivity and negative permeability,reverse Doppler effect and so on.Since the concept of metamaterials has been put forward,it becomes an important research direction in scientific research.Electromagnetic metamaterial absorbers and electromagnetic metamaterial polarization converters become a research hotspot as a branch in recent years.In this paper,an ultra-broadband metamaterial absorber,an ultra-broadband reflective electromagnetic metamaterial polarization converter and a broadband electromagnetic metamaterial linear-circular polarization converter were designed.In Part I of this paper,an ultra-wideband electromagnetic metamaterial absorbers with lumped resistance was designed.Its unit cell structure consisted of three layers of metal layers connected by five metal columns separated by two layers of FR4 dielectric substrate.The lumped resistance load on the top metal layer.Its absorptive bandwidth is 5.55-12.98 GHz.Its absorptive bandwidth covers the C-band and the X-band,and the absorption efficiency can reach more than 90% in the operating band.The designed absorber is insensitive polarization and wide-angle.For ideal absorbers,the absorbance curve should be a rectangle,and the absorbance curve of the designed absorber has a good selectivity and a good rectangular coefficient.The part II of this paper is researches on the absorption illusion caused by polarization conversion.The absorption illusion was found in the process of research on metamaterial absorbers.I mistakenly believe that changing the symmetry of the structure can broaden the bandwidth of the absorber.In fact,the changed structure does not absorb the electromagnetic wave,but changed the polarization of the electromagnetic wave.The fundamental reason for the absorption illusion is the polarization conversion.In Part III of this paper,an ultra-broadband reflective electromagnetic metamaterial polarization converter was designed.The unit cell of the polarization converter consisted of two overlapping metal rings and two metal short lines.Its substrate was F4 B with dielectric constant 2.2.Its polarization conversion rate could achieve more than 90% in 8.6-20.6GHz and its relative bandwidth was 82.2%.The reflective ultra-broadband electromagnetic metamaterial polarization converter was fabricated and measured.The measured results were in good agreement with the simulation results.In Part IV of this paper,electromagnetic metamaterial linear-circular polarization converters were designed.Firstly,a narrowband linear-circular polarization converter was designed.The relative bandwidth of 1dB axial ratio is 14.8%.In 16.1-18.7GHz,the linear polarization can be effectively converted into circular polarization.Then,a broadband linear-circular polarization converter was designed based on the narrowband linear-circular polarization converter.the bandwidth was broadened effectively by the slit,and it can be a large angle of incidence.Finally,I studied its polarization conversion mechanism.By adjusting the relevant parameters,the linear-circular polarization converter make the phase difference of the electric field component of the reflected wave satisfy the condition of the circularly polarized wave and realize the line-to-circular polarization conversion.